Identification and implementation of clean, sustainable, and affordable energy sources is of paramount importance for both environmental and human health concerns in the future. Sustainable hydrogen-based economies require that hydrogen production from protic media, such as H2O or mineral acids (HX), be coupled to solar power. Closed catalytic cycles for solar-to-hydrogen energy conversion from protic media must meet two technological hurdles: 1) proton reduction to generate dihydrogen, and 2) anion oxidation. A closed cycle for generating H2 from HX requires both proton reduction to form H2 as well as halide oxidation to generate X2. Thus far, molecular catalysts for authentic HX splitting have not been developed. Proof-of-concept systems, which employ chemical traps to promote X2 formation have been developed, however, these systems are not amenable to use in energy storage. This proposal aims to develop inexpensive catalysts based on earth- abundant, first-row transition metals for photochemical HX splitting. If successful, the proposed research would constitute a closed solar-to-hydrogen conversion scheme, in which mineral acids (HX) are converted to H2 and X2 using solar power. The resultant hydrogen could be used as fuel, either by sequestration as a liquid fuel by hydrogenation of small molecule substrates such as CO2 or by being used directly in hydrogen fuel cells. To accomplish the goal of developing a solar-to-hydrogen scheme, low-valent transition metal complexes, capable of proton reduction will first be developed. Simultaneously, investigations of metal dihalide complexes will allow critical requirements for halide oxidation to be defined. To enable the challenging, but necessary, X2 photoelimination, ligands wil be designed to provide access to highly oxidizing excited states. Studies of each of the fundamental steps of HX splitting - proton reduction and halide oxidation - will elucidate fundamental parameters required for each of the constituent half reactions. Informed by the requisite ligand features for each of the fundamental steps of HX splitting, transition metal catalysts will be designed to enable authentic HX splitting photocatalysis to be achieved.